Hitherto, in various fields including household appliances, building materials, vehicles, and the like, surface-treated steel sheets (electrogalvanized steel sheets) having electrogalvanized layers have been used. In recent years, there is a demand for further enhancement in corrosion resistance for an electrogalvanized steel sheet.
As a method of enhancing corrosion resistance of the electrogalvanized steel sheet, increasing the coating amount (coating weight) of a zinc-coated layer has been considered. However, in a case where the coating weight of the zinc-coated layer is increased, manufacturing cost is increased, and workability or weldability is degraded.
As a method of enhancing the corrosion resistance or appearance of an electrogalvanized steel sheet, a technique of forming a painted film on the surface has been widely used hitherto (for example, Patent Document 1). However, when adhesion (painting adhesion) between a coated layer and the painted film of the electrogalvanized steel sheet is insufficient, even though the painted film is formed on the surface thereof, an effect of forming the painted film is not sufficiently obtained. Therefore, enhancing painting adhesion as well as enhancing corrosion resistance of the electrogalvanized steel sheet is required.
In a case where the painted film is formed on the surface, increasing the thickness of the painted film is considered in order to enhance corrosion resistance. However, since a conventional inorganic film containing a silane coupling agent and the like, which has been widely employed as the painted film, does not contain a resin component, it is difficult to increase the film thickness.
In addition, there is a demand for enhancing conductivity (hereinafter, referred to as grounding properties) as well as corrosion resistance as described above for the electrogalvanized steel sheet on which the painted film is formed. Although thinning the painted film is effective as measures of enhancing conductivity, when a thin film is formed as described above, an enhancement of corrosion resistance cannot be achieved.
As described above, according to the related art, even when a film is further formed on the surface of the electrogalvanized steel sheet, it is very difficult to satisfy characteristics of both corrosion resistance and conductivity.
In addition, in a case where a painted film is formed on the surface of a coated layer having a high surface roughness, corrosion resistance is degraded. The roughness of the coated layer is significantly dependent on the roughness of a steel sheet before being subjected to coating. Therefore, in the electrogalvanized steel sheet according to the related art, when the roughness of the steel sheet is high, the roughness of the coated layer is necessarily increased, resulting in a deterioration of corrosion resistance. In order to prevent the deterioration of corrosion resistance, reducing the roughness of the coated layer in order to improve the roughness of the steel sheet is considered. However, this is not preferable in terms of manufacturing cost. Therefore, recently, there is a demand for enhancing corrosion resistance without dependence on the roughness of a steel sheet.
In addition, in recent years, there is a surface-treated steel sheet on which a chemical conversion treatment layer is formed by performing a chemical conversion treatment on the surface of a metal sheet and a painted film is formed by applying a paint onto a chemical conversion treatment layer. However, it is difficult to enhance corrosion resistance while ensuring workability even with this painted film.
Regarding the above described, in recent years, enhancing corrosion resistance by containing a vanadium element into a zinc-coated layer of a surface-treated steel sheet subjected to electrogalvanizing has been examined. For example, in Non-Patent Documents 1 to 4, techniques of causing the surface of a copper sheet as a cathode to have complex electro deposition of a Zn—V oxide are described.
However, in a case where a coated layer containing zinc and oxides of vanadium is formed on the surface of a steel sheet by an electro coating method using the techniques described in Non-Patent Documents 1 to 4, cracks are likely to occur on the surface of the coated layer. When cracks are formed on the surface of the coated layer, a surface-treated steel sheet having sufficient corrosion resistance is not obtained.